1. Technical Field
This invention relates generally to steel-backed bronze powder metal engine bearings.
2. Related Art
In the manufacture of copper-based engine bearings and bushings (collectively bearings), one common approach to bonding a copper-based functional layer to the steel backing is by a process which involves applying a single layer of bronze bearing material onto a moving backing strip which is subsequently sintered, cooled, roll compacted and then post-sintered. An overlay, such as tin or lead-tin-copper is often applied by electroplating or physical vapor deposition over the sintered bronze bearing layer to achieve good conformability and embedability properties of the bearing. Another approach to bonding the bronze functional layer is to cast such a layer onto a moving steel backing strip from a molten source, after which the cast functional layer can receive an overlay in a separate electro- or physical-vapor deposition process to enhance conformability and embedability of the bearing.
One inherent drawback to each of the above alternatives for preparing copper-based functional layers is that they are limited to application of a single copper-based alloy material. Depending upon the requirements of a given application, the chemistry of the powder metal or cast bronze functional layer is adjusted, such that the entirety of the functional layer is made up of the same alloy material.
With an ever-increasing demand on the load and wear requirements of bearings and the desire to minimize usage of certain elements, such as lead, the cost of preparing copper-based substitute alloys that satisfy all of the requirements of modern bearing and bushing applications becomes increasingly more challenging, particularly since the functional layer is typically the thickest of the layers applied to the steel backing.
What the inventor of the present application has recognized is that in many applications, it would be satisfactory to provide only a portion of the functional layer with a high performance/high cost copper-based alloy material, whereas the balance of the material of the functional layer could be of a lesser performance and less costly material, but for the limitations of the current processes available for preparing single layer bronze functional layers on steel backings. It is, accordingly, an object of the present invention to provide a method of forming multi-layer bronze functional layers on steel backings of bearings.
A method of fabricating multiple layer powder metal copper-based bearings according to a presently preferred process of the invention involves laying down a first layer of copper-based powder metal alloy on a metal backing strip and, without substantially densifying the first layer, laying down at least a second layer of copper-based powder metal alloy on top of the first layer, with the at least second layer being of different composition than that of the first layer. Further without substantially densifying the layers, the layers are subjected to a first sintering step after which the layers are cooled and then compaction rolled against the metal backing strip to densify and bond the layers to the backing strip and one another. Following compaction rolling, the layers are subjected to a second sintering step.
The inventor has found that by applying a first copper-based powder layer onto the steel substrate and thereafter applying at least a second layer of copper-based powder metal bearing material onto the first layer without substantially densifying the first layer followed by a first sintering and cooling operation after which the layers are roll bonded and subsequently sintered, that a multi-layer copper-based powder metal functional bearing material can be efficiently bonded to one another and to the steel backing, with the freedom to select the desired chemistries of the first and second layers in order to tailor the end properties of the functional layer.
This invention has the advantage of providing the ability to use a less expensive copper-based material for one layer in combination with a more expensive material for the remaining layer or layers.
The invention has a further advantage of being readily adaptable to existing sintering lines with little modification.
According to a particular feature, the first layer can be fabricated from relatively inexpensive copper-tin powder metal alloy and be made relatively thick, whereas the second layer can be fabricated of copper-tin-bismuth, or another specialty powder metal alloy, that serves as a substitute for lead-containing bronzes.